167 - CFD for consequences and fire growth with Jonathan Hodges

WEBVTT

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<v Wojciech Wegrzynski>Hello everybody, welcome to the Fire Science Show.

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<v Wojciech Wegrzynski>The mission of this podcast is to bring fire science to everyone, but it's not just direct fire science that we're talking about in here, and in this episode we're gonna talk about how fire safety engineering is being practiced and how fire science is actually used as a tool supporting fire safety engineering.

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<v Wojciech Wegrzynski>I may say In the previous episodes I had some of my colleagues who are in the same business as I am at ITB, me and my team are doing CFD analysis for a lot of projects.

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<v Wojciech Wegrzynski>We're using computational tools to design smoke control, assess the mobility in buildings, and that's the majority of my everyday work, and I like to meet with colleagues who do similar things at other companies, similar things at other companies.

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<v Wojciech Wegrzynski>This time I have Jonathan Hodges from Jensen Hughes company in the podcast and Jonathan is a research leader at Jensen Hughes.

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<v Wojciech Wegrzynski>He's a recipient of the SFPE 535 award Congratulations, jonathan, great job.

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<v Wojciech Wegrzynski>And also he's well known for being a very skilled CFD engineer and someone who's basically helping all the colleagues around the global office of Jensen Hughes in applying CFD in their everyday job.

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<v Wojciech Wegrzynski>So in this discussion we're not just going to talk about how CFD is used by, but about some clever ideas on how it can be used better.

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<v Wojciech Wegrzynski>What are the restrictions for using it?

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<v Wojciech Wegrzynski>How can we use CFD to iteratively generate our design fires to perhaps yield better simulations?

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<v Wojciech Wegrzynski>And, if you stay with us till the end, we have some predictions, or our opinions, of how the future of fire safety modeling will look like.

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<v Wojciech Wegrzynski>So, hopefully, a very interesting, insightful and very practical episode of the Fire Science Show.

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<v Wojciech Wegrzynski>I'm sure you'll enjoy it.

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<v Wojciech Wegrzynski>So let's spin the intro and jump into the episode show.

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<v Wojciech Wegrzynski>I'm sure you'll enjoy it.

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<v Wojciech Wegrzynski>So let's spin the intro and jump into the episode.

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<v Wojciech Wegrzynski>Welcome to the Firesize Show.

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<v Wojciech Wegrzynski>My name is Wojciech Wigrzyński and I will be your host.

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<v Wojciech Wegrzynski>This podcast is brought to you in collaboration with OFR Consultants.

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<v Wojciech Wegrzynski>Ofr is the UK's leading fire risk consultancy.

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<v Wojciech Wegrzynski>Its globally established team has developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and environment.

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<v Wojciech Wegrzynski>Established in the UK in 2016 as a startup business of two highly experienced fire engineering consultants, the business has grown phenomenally in just seven years, with offices across the country in seven locations, from Edinburgh to Bath, and now employing more than a 100 professionals.

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<v Wojciech Wegrzynski>Colleagues are on a mission to continually explore the challenges that fire creates for clients and society, applying the best research experience and diligence for effective, tailored fire safety solutions.

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<v Wojciech Wegrzynski>In 2024, ofr will grow its team once more and is always keen to hear from industry professionals who would like to collaborate on fire safety futures this year, get in touch at OFRConsultantscom.

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<v Wojciech Wegrzynski>Hello everybody, welcome to the Fire Science Show.

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<v Wojciech Wegrzynski>I am here today with Jonathan Hodges, the Director of Modeling at the Research Division of Jensen Hughes.

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<v Wojciech Wegrzynski>Hey, jonathan, good to have you in the podcast.

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<v Jonathan Hodges>Thanks for hosting me.

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<v Jonathan Hodges>Great to be here.

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<v Wojciech Wegrzynski>Yeah, congratulations on your 535 award, well deserved, mate.

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<v Jonathan Hodges>Thank you, I appreciate that.

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<v Wojciech Wegrzynski>So, jonathan, we've talked briefly at SFP Copenhagen.

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<v Wojciech Wegrzynski>I've read your recent papers on design fires.

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<v Wojciech Wegrzynski>It seems we are both doing very similar things in our companies and that is using modeling in our fire safety engineering, our fire safety engineering, and that would be the theme of the podcast.

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<v Wojciech Wegrzynski>While doing interviews with different people and talking about cfd and having conversations while traveling, I've noticed that cfd modeling means different things in different parts of the world.

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<v Wojciech Wegrzynski>Like poland is the car park country, we do cfd modeling for car parks all of them and I know in some parts of the world it's not that common.

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<v Wojciech Wegrzynski>I know some parts to uh atria and corridors.

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<v Wojciech Wegrzynski>I wonder what kind of CFD person are you and what kind of CFD modeling in fire safety engineering you're dealing with?

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<v Jonathan Hodges>So Jensen Hughes is a really big company.

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<v Jonathan Hodges>We do a lot of different modeling efforts.

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<v Jonathan Hodges>My team does a lot of performance-based design.

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<v Jonathan Hodges>Smoke control my team does a lot of performance-based design.

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<v Jonathan Hodges>Smoke control, emission of fireproofing alternative means in that space.

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<v Jonathan Hodges>We do a lot in the transportation sector for subway ventilation design, some in the car park, like you had mentioned.

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<v Jonathan Hodges>We also do a lot in battery energy storage systems and looking at explosion prevention as well as looking at separation distance from adjacent energy storage enclosures and make sure the whole system design when you've got multiples is not going to spread when the system's going off.

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<v Wojciech Wegrzynski>Having offices around the world.

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<v Wojciech Wegrzynski>Do you also see those differences between countries how people apply CFD?

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<v Wojciech Wegrzynski>Does it mean different things in different J&K offices?

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<v Jonathan Hodges>It does.

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<v Jonathan Hodges>We don't do a lot of high-rise timber, for example, in the US, but in our European offices.

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<v Jonathan Hodges>That is more commonly used.

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<v Jonathan Hodges>So we'll have teams in our European offices who are doing those types of analyses.

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<v Jonathan Hodges>But we actually, even though we are a global company, we are pretty well connected and organized.

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<v Jonathan Hodges>I host office hours for all the FDS modeling people in our team in the company, modeling people in our team in the company.

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<v Jonathan Hodges>So we've got about an hour a week set aside where we talk about FDS modeling and how we are using the tools to make sure we're improving consistency.

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<v Wojciech Wegrzynski>And FDS is the main tool used by the company.

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<v Jonathan Hodges>It's the main CFD tool that we use.

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<v Jonathan Hodges>We also use FLAX for some of the deflagration stuff in the energy storage systems, as well as Fluent or some of the general CFD when we're looking at air conditioning performance or cooling in electrical spaces, that kind of thing.

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<v Jonathan Hodges>But then we also use a lot of zone models, like we'll use CFAST as well as.

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<v Jonathan Hodges>Ses the subway environment simulator that we use a lot for the transportation sector.

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<v Wojciech Wegrzynski>That's interesting because in many places of the world, like in Poland, engineering design in fire safety would be largely synonymous with CFD simulations.

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<v Wojciech Wegrzynski>Like, I wouldn't say I've seen any study with zone modeling in the past five years like a zone model focused study.

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<v Wojciech Wegrzynski>I haven't seen any content in Poland.

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<v Wojciech Wegrzynski>I haven't seen SES.

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<v Wojciech Wegrzynski>Yeah, we had some coming with engineering teams from outside of Poland to Polish tunneling projects, but still they've learned a hard lesson that the Polish firefighter the one that has authority in here they want colorful images and CFD was necessary for those projects as well.

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<v Wojciech Wegrzynski>So quite, quite interesting, interesting.

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<v Wojciech Wegrzynski>And is it a part of performance-based design regime?

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<v Wojciech Wegrzynski>How does it work for you, at least in the us side?

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<v Jonathan Hodges>so typically the zone fire models are not used as much in specific applications.

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<v Jonathan Hodges>I mentioned at the start those we do when you're looking at naval systems uh, okay, at ships where you have compartmentalization, and so we do a lot of zone fire modeling in that space as as well as in nuclear power plants You've got the detailed fire modeling to look at equipment failure from far targets, looking for separation of systems for safe shutdown.

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<v Jonathan Hodges>But a lot of those analyses as a first cut are done either with spreadsheet tools or zone fire models, and then those are very conservative models and then that are very conservative and then when you're seeing event that's very bad, you may then dive deeper into the cfd modeling to understand those, because just there's too many to evaluate with cfd so let's try to go deeper into how cfd is performed.

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<v Wojciech Wegrzynski>It's also performed differently.

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<v Wojciech Wegrzynski>Do you have a very specific routine like?

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<v Wojciech Wegrzynski>Do you follow any specific guidance or you build up your internal guidance on how to put CFD as a part of performance-based design, because I assume that's also important for you?

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<v Jonathan Hodges>It depends on the application.

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<v Jonathan Hodges>We do have our general principles.

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<v Jonathan Hodges>We have engineering guidance on how to use FDS as a company, as well as general guidance on Contam and a couple others, and so we do have principles there of things that you should always be doing how to define boundary conditions and when you're reviewing someone's work, what is included in that review, things that you definitely need to be looking at the checklist.

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<v Jonathan Hodges>So we have a lot of that kind of guidance for specific projects, especially in the design space.

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<v Jonathan Hodges>We're usually working within some code framework, and so we start with understanding what is the requirement based on the code and then use that to design what we're going to be evaluating with the CFD model.

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<v Wojciech Wegrzynski>With the CFD models and I know that this is important for you as well, because we've discussed this previously.

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<v Wojciech Wegrzynski>Basically, what you put in is what you get out.

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<v Wojciech Wegrzynski>Right.

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<v Wojciech Wegrzynski>And I liked I think it was an interview with Mike Spearpoint when he introduced me to the concept of consistent level of crudeness, that the weakest part in your simulation.

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<v Wojciech Wegrzynski>Like there's no point of running extremely complicated modeling if your input is extremely base and uncertain.

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<v Wojciech Wegrzynski>Right, if you completely make up your fire, what's the point of having a really complicated fire model to solve that if this consistent level of crudeness is not maintained in your simulations?

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<v Wojciech Wegrzynski>So how do you choose the input for your simulations, what steps do you take and how does it look like?

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<v Jonathan Hodges>So I think that goes to the difference between fire effects modeling and fire growth modeling those as an industry, where the cfd models that we use for where you've prescribed your heat release rate, your smoky so it yields and your toxic product yields, and then looking at where it's going within your space, are pretty robust, especially when you're looking in the kind of far field.

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<v Jonathan Hodges>You can look at the validation basis in FDS and see that does a pretty good job in those spaces.

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<v Jonathan Hodges>Where we've seen a lot of people really trying to push the envelope lately is in fire growth modeling, where you're trying to, instead of using a prescriptive design fire or using specific test data, trying to use the model to predict the design fire, to either scale it up in some way or to swap out materials, that kind of thing.

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<v Jonathan Hodges>And there you're stacking uncertainties on top of each other because now you're trusting that your uncertainty in the modeling parameters as well as the material properties and the model physics aren't running away with those interactions, and you see this a lot.

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<v Jonathan Hodges>I think your example of the consistent level of crudeness is a good one.

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<v Jonathan Hodges>Especially in something like the batteries world You'll see people who are trying to do detailed chemical modeling of the battery and thermal modeling of the insides of the battery, and then we're using that to try and come up with how it's progressing in its thermal runaway and when you're going to get off gassing.

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<v Jonathan Hodges>So you've got a lot of complex physics that we try to embed in that and then using a very coarse simulation of where it's going to go within the space and not looking at the detailed reaction kinetics of what's going on in the gas phase.

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<v Jonathan Hodges>So you're doing a lot of good work on the chemistry side, but it's not the level of fidelity there and then what they're using in the gas phase are inconsistent.

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<v Wojciech Wegrzynski>The idea that designed fire is an output came out of Jose Toretto.

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<v Wojciech Wegrzynski>I think he formulated that at some point.

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<v Wojciech Wegrzynski>That designed fire is actually an outcome of a fire, and for me it's an interesting concept because I kind of get the interactions you would get in a fire scenario.

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<v Wojciech Wegrzynski>Okay, if you're doing a very simple fire and a very simple fire for me would be a ventilation-controlled compartment fire, you know, fully flashoovered, only the uh heat transfer at the walls and the air coming in, flames coming out.

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<v Wojciech Wegrzynski>That's the only things that that you know exchange the heat and mass in your model.

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<v Wojciech Wegrzynski>For me that is a simple fire because I there's not that many thing you know that would drive it.

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<v Wojciech Wegrzynski>If you think about the models or the fires that we would deal with in our engineering, it's not fully grown flash-overed fires that we would deal with in our engineering.

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<v Wojciech Wegrzynski>It's not fully grown flash-over fires.

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<v Wojciech Wegrzynski>Usually we would rather, like I always say, the fire engineering is for the first phase of fire, for the growth phase, because that's where you can take actions, that's where you need engineering.

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<v Wojciech Wegrzynski>Once you get into fully developed fire, that's where you need firefighters and fire resistance.

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<v Wojciech Wegrzynski>Right, I wonder, like is the fire spread modeling?

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<v Wojciech Wegrzynski>Is it even possible at this point for the engineering purpose.

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<v Wojciech Wegrzynski>What was your opinion on that?

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<v Jonathan Hodges>I think it's possible to be using these tools to inform design.

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<v Jonathan Hodges>It's not necessarily, I think, we need to do.

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<v Jonathan Hodges>When you're looking at fire spread, you need to have a grid convergence.

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<v Jonathan Hodges>We're running at least at a few grid resolutions.

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<v Jonathan Hodges>We'll typically do three, starting with one that's kind of an engineering scale where we think it should be based on our guidance, and then one that's a factor two larger and one that's a factor two lower.

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<v Jonathan Hodges>So you can look at the sensitivities and then use that to look at flame spread rates.

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<v Jonathan Hodges>Heat fluxes use that to inform how we're building the design fluxes.

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<v Jonathan Hodges>Use that to inform how we're building the design.

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<v Jonathan Hodges>One example that we had was we were doing some design in like an amusement park type area and they had plastic or polymer of some sort water slide type thing and they wanted to understand how big of a fire could this thing produce, since we're looking at some interior spaces and nobody's burned a full water slide without the water on it, and so you have to come up with some design profile to be using for that.

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<v Jonathan Hodges>You could do a surface area calculation for the whole thing and then do some linear flame spread rate and then do a heat release rate per unit area based on that, and that might be an OK starting point.

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<v Jonathan Hodges>But you also, when you actually burn these things, you can start getting dripping.

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<v Jonathan Hodges>It's not going to stay clean like it would in a model.

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<v Jonathan Hodges>So we came up with conservative assumptions.

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<v Jonathan Hodges>Use fire growth modeling to evaluate what's a reasonable flame spread rate.

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<v Jonathan Hodges>When you start looking at these complex shapes, where you're not necessarily just in a lift configuration or a horizontal configuration, use that to get a realistic idea of the flame spread rate and then prescribe that in the model.

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<v Jonathan Hodges>So we describe what the fire scenario is, and then we've defined our fire scenario now and then prescribe that in the model, where then we're looking at the fire effects from that scenario that we've defined.

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<v Wojciech Wegrzynski>So, in other words, it would not be a spread modeling like you have one model, you just start the fire, it spreads.

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<v Wojciech Wegrzynski>It's more like using simulation to inform decisions and refine the scenario until you reach something that you are comfortable with.

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<v Wojciech Wegrzynski>Yeah, kind of in between of the worlds.

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<v Wojciech Wegrzynski>That's an interesting approach.

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<v Wojciech Wegrzynski>Do you see that applied elsewhere?

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<v Wojciech Wegrzynski>Because, okay, the scenario scenario with the slide is very peculiar.

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<v Wojciech Wegrzynski>But in spaces like car parks or For me, car parks are interesting because I understand the importance of the height of the ceiling, that we've done a lot of parametric research that's shown us that this is the most important variable, at least if we're considering the life safety in car park.

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<v Wojciech Wegrzynski>And yet in a car park you would have very specific, very strongly prescribed fire curves, you know, up to a point where they are favorites of people, like I have my favorite curve for a car park, right.

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<v Jonathan Hodges>Yet.

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<v Wojciech Wegrzynski>I imagined in every car park this would look differently.

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<v Wojciech Wegrzynski>Do you think such a refined approach here would be something of use?

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<v Jonathan Hodges>Cars in particular are very hard to model.

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<v Jonathan Hodges>I'm sure you're aware of that because you've got a lot of non-combustibles surrounding your combustibles and getting the thermal model right on that is really complicated.

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<v Jonathan Hodges>So I don't necessarily think we're at a place where you can really be looking at flame spread in that way.

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<v Jonathan Hodges>Now you could be looking at you have a sign fire that you're really competent in for your first vehicle and be using that to be looking at heat fluxes to adjacent vehicles.

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<v Jonathan Hodges>But the approach that I would be using in that would be more related to okay, what does the literature say on critical heat flux for ignition of my adjacent vehicle, and then be doing that as something as a hand calc or doing it as an iterative CFD design where we're predicting the heat flux and then saying, okay, now the next one's going to ignite at this time, and then doing it that way rather than trying to actually predict the heat release rate of that individual vehicle.

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<v Jonathan Hodges>But that does open a can of worms if you start to look into it, because if you have just one car burning and you measure the heat release rate, that's fine, you can look at that.

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<v Jonathan Hodges>But then if you have two cars burning now, does the second car burn at the same rate as that first car, or do you have an additional accelerant due to the additional heat being added by the adjacent vehicle?

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<v Jonathan Hodges>But what about when your third vehicle does it burn at the same rate as the first one?

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<v Wojciech Wegrzynski>Yeah, what about in the seventh right?

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<v Jonathan Hodges>Yeah, and so that gets to one of the things that we'll talk about today that scaling up of experimental data, where we need to understand how do you take data that's something that you can get in a lab from testing and scale that up to a realistic scenario where your conditions might not be the same as what you have in the lab.

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<v Wojciech Wegrzynski>And what about the other end of the universe?

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<v Wojciech Wegrzynski>You said you would separate the fire modeling into effects and fire growth modeling.

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<v Wojciech Wegrzynski>So if you model fire effects, there are some design fires which are like.

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<v Wojciech Wegrzynski>They're not even experimental, they're like just magic numbers that came out of thin air.

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<v Wojciech Wegrzynski>In Poland our magic number for a long time, and still is in many cases would be 2.5 convective.

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<v Wojciech Wegrzynski>So that's like 3.25 total heat release rate and just alpha T squared that fast until it reaches that and you're good For a lot of designs and commercial spaces.

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<v Wojciech Wegrzynski>You have a magic number that would be a fan favorite in the US.

00:17:45.361 --> 00:18:11.314
<v Jonathan Hodges>So the historic guidance was 5 megawatts or 5,200 megawatts Sorry, 5,200 kilowatts was the prevailing guidance for many years and I think that has been shrinking over time as people are using test data for something that's sprinkler controlled, and if you're looking at a balcony spill plume or something and you're crediting sprinklers, it's probably something around like 1.5 megawatts is pretty typical for that.

00:18:11.779 --> 00:18:35.778
<v Jonathan Hodges>Whereas for an axisymmetric plume typeks if you're looking like in a building or you're looking at Christmas trees or like upholstered furniture, that kind of stuff, and you can always come up with some combination of those things being close to each other.

00:18:35.778 --> 00:18:37.301
<v Jonathan Hodges>That'll get you to that number.

00:18:37.301 --> 00:18:42.084
<v Jonathan Hodges>But it doesn't necessarily mean that that's the conservatively bounding scenario in all cases.

00:18:43.971 --> 00:18:49.002
<v Wojciech Wegrzynski>But I don't think there's strong experimental evidence for any of these particular numbers.

00:18:49.002 --> 00:18:53.894
<v Wojciech Wegrzynski>I think they're just at the scale of what you would see Like if you go NIST calorimetry.

00:18:53.894 --> 00:18:59.055
<v Wojciech Wegrzynski>You can have a loft seat, you can have a couch, there is a kiosk.

00:18:59.055 --> 00:19:01.557
<v Wojciech Wegrzynski>I think there was an office configuration.

00:19:01.557 --> 00:19:06.710
<v Wojciech Wegrzynski>They used the office configuration for World Trade Center as well, so they had some office configurations.

00:19:06.710 --> 00:19:09.332
<v Wojciech Wegrzynski>Of course, christmas trees go Maryland.

00:19:09.332 --> 00:19:17.194
<v Wojciech Wegrzynski>They have endless collection of burned down Christmas trees, which actually shows you how varied it can be right.

00:19:17.194 --> 00:19:23.232
<v Wojciech Wegrzynski>It's so varied to a point we have a competition to guess the number before we burn the Christmas tree.

00:19:23.232 --> 00:19:25.758
<v Wojciech Wegrzynski>That's how variable it is.

00:19:25.758 --> 00:19:27.200
<v Wojciech Wegrzynski>It's not a single number.

00:19:27.200 --> 00:19:30.085
<v Wojciech Wegrzynski>That is going to be two and a half megawatts, right, or five.

00:19:30.085 --> 00:19:34.461
<v Wojciech Wegrzynski>There's no strong experimental evidence that it is five.

00:19:34.461 --> 00:19:42.618
<v Wojciech Wegrzynski>It's just a choice someone made a long time ago and it propagated everywhere and now, out of convenience, we're using that.

00:19:43.010 --> 00:19:46.740
<v Jonathan Hodges>It's also in some ways an artifact of the labs that we have.

00:19:46.740 --> 00:20:04.843
<v Jonathan Hodges>There's very few labs that can measure heat release rates higher than five megawatts, and so you know, when we're burning stuff in our lab, we kind of design what we're going to test so that it is not going to be larger than our hood can handle, and so if your hood can only handle five megawatts, you're not going to be testing things that are higher than five megawatts.

00:20:04.843 --> 00:20:27.519
<v Jonathan Hodges>And so there's some artifacting there that you're selecting things that you can measure, and when you start to get larger than that with like rail, car fires or vehicles or heavy good vehicles, and you start putting things in tunnels or other things, now you get a lot more geometric effects and ventilation effects that are affecting your measurements and a lot more uncertainties associated with it.

00:20:28.131 --> 00:20:36.217
<v Wojciech Wegrzynski>Some time ago I've put forward in Poland an idea that I see some value in that, even that it's artificial number.

00:20:36.217 --> 00:20:44.978
<v Wojciech Wegrzynski>If you do that on multiple buildings, if you do that a hundred times at least, it becomes some sort of a benchmark.

00:20:44.978 --> 00:20:51.801
<v Wojciech Wegrzynski>You know, a benchmark test, like in a way the ISO curve we use for standard fire resistance testing.

00:20:51.801 --> 00:21:09.813
<v Wojciech Wegrzynski>Like we know it's bullshit, like we know it's representative of a very small portion of fires, right, but nevertheless it became a benchmark in assessing the fire resistance and you at least can refer the fire resistance to this standard test and compare worldwide In a similar way.

00:21:09.813 --> 00:21:33.490
<v Wojciech Wegrzynski>For me, a very specific two and a half five megawatt design fire like perhaps it's not going to tell me everything about the performance in my building, for sure it's not going to show me the behavior of the building as a response to real fire, but at least I can tell okay, this atrium is so much better than the one I worked at a year ago Because with the same design fire I see completely different outcomes.

00:21:34.171 --> 00:21:55.855
<v Jonathan Hodges>I would agree with that because it does let you compare apples to apples across different designs and structures, and the difficulty you run into with the US or really any municipality you're going to have lots of different designers and consultants who are doing these analyses and because the prescriptive design opens up and allows the designer to choose a reasonable design fire.

00:21:55.855 --> 00:21:58.500
<v Jonathan Hodges>Some people are going to choose 2,500.

00:21:58.500 --> 00:22:00.392
<v Jonathan Hodges>Some people are going to choose 5,000.

00:22:00.392 --> 00:22:02.336
<v Jonathan Hodges>Some people are going to choose 1,000.

00:22:02.336 --> 00:22:23.378
<v Jonathan Hodges>So you get into a position where it's up to the engineers, almost their comfort level, with how conservative they want to be with the design fire, and it's just a difficulty because no one really wants to prescribe what design fire you need to be using in a situation they want to put that on to the engineer to decide.

00:22:23.800 --> 00:22:56.040
<v Wojciech Wegrzynski>But then because of that you get a lot of variability in what the engineers decide to use of design fires, boundary conditions, things that you put into your model and then becomes just a standard test that you could compare against a bunch of other results that you have.

00:22:56.040 --> 00:23:08.638
<v Wojciech Wegrzynski>You're an AI expert, so you know that better than most of us that if you would have such a database, it would allow for quick comparison, a robust assessment across a large database of outcomes.

00:23:08.638 --> 00:23:20.300
<v Wojciech Wegrzynski>Where does this outcome place in the ladder of or whatever, and performance based design CFD, where you would go back into engineering your design fire, and that being a part of the PBD task itself.

00:23:20.300 --> 00:23:22.693
<v Wojciech Wegrzynski>I think that this would be interesting times.

00:23:22.693 --> 00:23:24.778
<v Wojciech Wegrzynski>I think we'll see that in the future.

00:23:24.959 --> 00:23:25.359
<v Jonathan Hodges>I agree.

00:23:25.359 --> 00:23:36.701
<v Jonathan Hodges>And you look at round robin studies where they'll have, you know, send out the same prompt to different firms and ask them to all design the same thing or measure the same heat release rate per unit area in a cone.

00:23:36.701 --> 00:23:44.411
<v Jonathan Hodges>You see a lot of these kinds of things where you get to see that variability and it always comes back as highly variable in whatever case you look at.

00:23:44.411 --> 00:23:51.134
<v Jonathan Hodges>You can even look at the SFBE PBD conference that we were at earlier this year that all the PBD design examples.

00:23:51.655 --> 00:24:05.823
<v Jonathan Hodges>We saw a lot of diversity in the different design solutions, so we don't want to cut out the creativity of the industry and being able to come up with unique solutions, but having standard benchmarks, I think, is a reasonable way to do it.

00:24:06.269 --> 00:24:07.480
<v Wojciech Wegrzynski>Actually for the round robins.

00:24:07.480 --> 00:24:16.135
<v Wojciech Wegrzynski>I remember an exercise so I'm in the European Commission that writes the standard CN and we're working on part five, which is for smoke control.

00:24:16.135 --> 00:24:21.461
<v Wojciech Wegrzynski>And there was this exercise that some people did about simulating balcony spill plume.

00:24:21.461 --> 00:24:23.215
<v Wojciech Wegrzynski>I remember that case study.

00:24:23.215 --> 00:24:33.333
<v Wojciech Wegrzynski>That was a decade ago and we've agreed on very specific design, very specific boundary conditions, like we've agreed on everything you would put into the model and we've run it as.

00:24:33.333 --> 00:24:47.518
<v Wojciech Wegrzynski>Like our team did it in Ansys, some other guys did it in FDS, someone else did use, I think, jasmine and actually there was not that much scatter, like pretty much everyone got very similar results but everything was prescribed.

00:24:47.518 --> 00:24:52.555
<v Wojciech Wegrzynski>There was like no discrepancy, no choice From that exercise.

00:24:52.555 --> 00:24:56.435
<v Wojciech Wegrzynski>It was not published, it was just an exercise for the committee to see what's going to happen.

00:24:56.435 --> 00:25:04.079
<v Wojciech Wegrzynski>But you could see that you could potentially get into the place where this becomes a test, where this becomes reputable.

00:25:05.451 --> 00:25:11.712
<v Wojciech Wegrzynski>One more idea I also remember that from the discussions at the CEN, the robustness scenario.

00:25:11.712 --> 00:25:17.953
<v Wojciech Wegrzynski>I know in some places of the world if you allow PPD choice of the design, fire would still have a robustness check.

00:25:17.953 --> 00:25:24.496
<v Wojciech Wegrzynski>Let's say, one megawatt fire and assuming a failure of smoke control or something I think Swedish have it.

00:25:24.496 --> 00:25:31.474
<v Wojciech Wegrzynski>I think in New Zealand it exists in the CVM2 method Like a one megawatt firewood also always simulate.

00:25:31.474 --> 00:25:32.358
<v Wojciech Wegrzynski>What's your take on that?

00:25:32.559 --> 00:25:33.605
<v Jonathan Hodges>I think it's a good approach.

00:25:33.605 --> 00:25:54.278
<v Jonathan Hodges>You always want to make sure that if one system fails, you have some redundancies in place, and so in the US market we see this a lot in the transportation sector, where you're using jet fans to prevent back layering so that people can egress out of a stop rail car and we always assume that the fire.

00:25:54.278 --> 00:26:02.029
<v Jonathan Hodges>You know, we'll look at different scenarios, but if you have the fire located at one your most important jet fan, then that jet fan, it's failed.

00:26:02.510 --> 00:26:06.819
<v Jonathan Hodges>And then you we make sure that the system still works without that jet fan.

00:26:06.819 --> 00:26:09.461
<v Jonathan Hodges>And so we do that in all of our systems not designs in the transportation sector to make sure that the system still works without that jet fan.

00:26:09.461 --> 00:26:16.076
<v Jonathan Hodges>And so we do that in all of our systems not designs in the transportation sector to make sure that, even if you're losing one of your key systems, that it's still going to be okay.

00:26:16.851 --> 00:26:17.955
<v Wojciech Wegrzynski>Using like a one megawatt.

00:26:17.955 --> 00:26:20.557
<v Wojciech Wegrzynski>I've also got a good example for that.

00:26:20.557 --> 00:26:23.118
<v Wojciech Wegrzynski>We use that design fire in metro systems.

00:26:23.118 --> 00:26:37.219
<v Wojciech Wegrzynski>More so when we were designing the corridors pretty much the concrete corridors which people use to reach metro Like there's nothing you can burn in a concrete corridor right Luggage trash bags, something like that, yeah, yeah, but how often you see burning luggage?

00:26:37.239 --> 00:26:50.762
<v Wojciech Wegrzynski>come on, only in a Swedish fire experiments you see burning luggage At that point and that was a heavy criticism to us Like why do you insist on having a fire in a place where there's no combustibles, like there's no fire?

00:26:50.762 --> 00:27:02.295
<v Wojciech Wegrzynski>Why do we have to put smoke extraction in a place where there is literally nothing to be burnt and there's stuff on the metro that will not allow to bring in combustibles and fast forward six, seven years?

00:27:02.295 --> 00:27:04.821
<v Wojciech Wegrzynski>We're living in the world of electric bikes, right?

00:27:04.821 --> 00:27:23.796
<v Wojciech Wegrzynski>Everyone carries a one megawatt fire source conveniently with them across the metro station nowadays and I'm super happy that we've insisted on that, because if we have not now, perhaps we would have a challenge to retrofit the station to adapt, adjust for the new challenge that has emerged, which we absolutely have not seen six or seven years ago.

00:27:23.796 --> 00:27:28.862
<v Wojciech Wegrzynski>So I'm in huge favor for those robustness scenarios.

00:27:28.862 --> 00:27:36.318
<v Wojciech Wegrzynski>Another thing let's go back to some blazing between fire effects and fire growth modeling.

00:27:36.318 --> 00:27:39.692
<v Wojciech Wegrzynski>What's the representative fire for the real world?

00:27:39.692 --> 00:27:43.405
<v Wojciech Wegrzynski>How would you interpret the representativeness of the fire?

00:27:43.630 --> 00:27:51.223
<v Jonathan Hodges>Let me ask you this clarifying question Are you asking for a prescribed heat release rate curve versus, like an alpha T squared type curve?

00:27:52.130 --> 00:28:16.178
<v Wojciech Wegrzynski>No, I would say, like you have an office and you would go okay, in this office, I would go with 3.5 megawatt fire because I find it representative that I see coming up a lot in CFD analysis Because honestly, the best would be to go multi-parametric and test 100 fires and see 100 outcomes right, but no one can afford that yet.

00:28:16.178 --> 00:28:21.558
<v Wojciech Wegrzynski>Perhaps with GPU and AI-assisted CFD, this conversation will be redundant.

00:28:21.558 --> 00:28:27.257
<v Wojciech Wegrzynski>And if you're listening to the podcast in 2030, just speed up 10 minutes because it's not going to be interesting to you.

00:28:27.257 --> 00:28:29.198
<v Wojciech Wegrzynski>But in 2024, it is.

00:28:29.198 --> 00:28:42.722
<v Wojciech Wegrzynski>So when you would say your fire is representative to a space, no matter if you just use an experiment, alpha, t-square or just best assumption of a megawatt, just say five megawatts is representative.

00:28:43.250 --> 00:29:00.134
<v Jonathan Hodges>So for something like an office space, the way I would typically handle that is I would assume that you've got enough fuel to reach flashover, and if it's not something that's going to have liquid hydrocarbons, then you don't typically see anything faster than a fast growth unless you've got liquid accelerants present in some way.

00:29:00.134 --> 00:29:06.576
<v Jonathan Hodges>So I would probably assume that it's a fast growing fire up to flashover conditions within the space.

00:29:06.576 --> 00:29:08.520
<v Jonathan Hodges>But this actually gets to.

00:29:08.520 --> 00:29:14.671
<v Jonathan Hodges>One of the things that I've talked about a lot with people in our company is that's not the peak heat release rate that you can get.

00:29:14.671 --> 00:29:31.869
<v Jonathan Hodges>That's the peak heat release rate that the oxygen coming into the door can support, but you can pyrolyze a lot more fuel than that in a space and so, depending on your local flame extinction and ventilation conditions in the hallway, you could get a lot more flaming.

00:29:32.330 --> 00:29:50.356
<v Jonathan Hodges>Or, if you've got exit signs or other potential sparking locations, if you had localized flame extinction you could still have ignition of the sunburned hydrocarbons that you're pyrolyzing and having come out, hydrocarbons that you're pyrolyzing and having come out, and so that's a.

00:29:50.356 --> 00:29:56.236
<v Jonathan Hodges>You know, if you're just looking at design within the room, I think those flashover correlations and the fast growth to that is a reasonable estimate to be using.

00:29:56.236 --> 00:30:19.105
<v Jonathan Hodges>But if you're using that to inform, like facade design or looking at smoke transport throughout the space, I think you also want to try and understand, you know, based on your fuel load density in that space, general heat of gasification of the materials, what would be the maximum pyrolysis rate you would expect to see and use that to come up with a design fire that includes these things.

00:30:20.211 --> 00:30:35.021
<v Wojciech Wegrzynski>What you described is some sort of maximum scenario, like if we're at the ventilation limited fire, that's probably the max you can get in a room and the growth rate, yeah, okay, the boundaries of that would be characterized by the physics of the flame spread.

00:30:35.021 --> 00:30:38.019
<v Wojciech Wegrzynski>David Morris's episode in the podcast, highly recommend it.

00:30:38.019 --> 00:30:40.939
<v Wojciech Wegrzynski>Go listen to that if you want to learn more about that.

00:30:40.939 --> 00:30:43.529
<v Wojciech Wegrzynski>But how probable is that scenario Like?

00:30:43.529 --> 00:30:49.123
<v Wojciech Wegrzynski>That would not represent the most probable fire, which would most likely be something localized right.

00:30:49.123 --> 00:30:53.382
<v Wojciech Wegrzynski>So it's always the maximum fire, the approach right.

00:30:54.051 --> 00:30:56.400
<v Wojciech Wegrzynski>I have these troubles when I'm doing designs.

00:30:56.400 --> 00:31:07.842
<v Wojciech Wegrzynski>Okay, in a confined space, I agree, because the likelihood that if it's not interrupted by a sprinkler or a person, it's most likely going to grow to a decent size.

00:31:07.842 --> 00:31:20.462
<v Wojciech Wegrzynski>But if you design, for example, a shop in a mall or a car park, again it's very unlikely that my fire would grow to 100 vehicles in my car park.

00:31:20.462 --> 00:31:24.260
<v Wojciech Wegrzynski>I'm not saying it's not possible because we've seen those fires.

00:31:24.260 --> 00:31:26.778
<v Wojciech Wegrzynski>The last one was a week ago in Korea, I believe.

00:31:26.778 --> 00:31:28.365
<v Wojciech Wegrzynski>Yet most of those fires would end up on.

00:31:28.365 --> 00:31:31.593
<v Wojciech Wegrzynski>One was a week ago in Korea, I believe, yet most of those fires would end up on one, two, three vehicles burned down.

00:31:31.593 --> 00:31:36.758
<v Wojciech Wegrzynski>So the likelihood of it growing out of control is also like a certain number.

00:31:36.758 --> 00:31:42.835
<v Wojciech Wegrzynski>To what extent my design fire should be representative of very low probability events?

00:31:42.835 --> 00:31:44.458
<v Wojciech Wegrzynski>How do you decide on that?

00:31:44.898 --> 00:31:48.032
<v Jonathan Hodges>That's a good question and it's a fire risk assessment.

00:31:48.032 --> 00:31:50.079
<v Jonathan Hodges>You have to decide likelihood and consequence.

00:31:50.079 --> 00:31:58.403
<v Jonathan Hodges>Do you design to the 95th percentile, the 99th percentile or the 99.99 percentile?

00:31:58.403 --> 00:32:01.210
<v Jonathan Hodges>And so that's a risk management decision.

00:32:01.210 --> 00:32:07.763
<v Jonathan Hodges>And that is when we go to probabilistic analyses, which is the direction that we're going with.

00:32:07.763 --> 00:32:40.074
<v Jonathan Hodges>When we go to probabilistic analyses, which is the direction that we're going with, like you had mentioned, once you have AI-driven CFD is prescribed at what risk they're allowed to take on, based on the Nuclear Regulatory Commission, and so there's an actual risk number.

00:32:40.134 --> 00:32:41.378
<v Jonathan Hodges>That's being regulated too.

00:32:41.378 --> 00:32:43.807
<v Jonathan Hodges>So it's a quantitative metric.

00:32:43.807 --> 00:32:51.577
<v Jonathan Hodges>You do your quantitative analysis, see what your risk is, and if your risk ever goes higher than that because of emergent events, then you need to be mitigating it in some way.

00:32:51.577 --> 00:33:11.736
<v Jonathan Hodges>But that's also a very regulated industry with very high controls on the spaces, whereas in your office space, if someone wanted to put 50 e-bikes in because they're trying to sell them on an online marketplace and they need to store them somewhere, they just have them all sitting in their office while they're waiting to sell them.

00:33:11.736 --> 00:33:14.310
<v Jonathan Hodges>There's nothing saying they can't do that.

00:33:14.310 --> 00:33:18.320
<v Jonathan Hodges>There might be city ordinances, but there's not rigorous controls to prevent that.

00:33:18.320 --> 00:33:26.650
<v Jonathan Hodges>That's a very low likelihood event, but it is something that could happen, and so we have to decide what is the worst case that we need to be able to withstand.

00:33:26.650 --> 00:33:27.030
<v Jonathan Hodges>What is the worst?

00:33:27.050 --> 00:33:27.872
<v Wojciech Wegrzynski>case that we need to be able to withstand.

00:33:27.872 --> 00:33:40.420
<v Wojciech Wegrzynski>I sometimes heard Wade describe it as the worst credible scenario, like the credibility here Okay, you brought an example of nuclear industry.

00:33:40.420 --> 00:33:46.854
<v Wojciech Wegrzynski>That's the credibility there would be defined by the risk value that's regulated by the authority, I would argue.

00:33:46.854 --> 00:33:59.097
<v Wojciech Wegrzynski>In tunnel space it's also fairly easy because in tunnel there's a high acceptance for risk-based engineering and basically that's the only way you can PPD a tunnel only through risk assessments.

00:33:59.097 --> 00:34:03.978
<v Wojciech Wegrzynski>But for a shopping mall or a car park, I wonder where those boundaries are.

00:34:03.978 --> 00:34:07.311
<v Wojciech Wegrzynski>Do you sometimes have those discussions with authorities Like?

00:34:07.311 --> 00:34:13.951
<v Wojciech Wegrzynski>I feel it would be some sort of agreement between the authority having a jurisdiction and an engineer doing the project.

00:34:14.753 --> 00:34:34.916
<v Jonathan Hodges>We do have those conversations and it does end up being a discussion where we say this is what we think is the worst credible event, that, based on the fuel sources, what's around, what we think it needs to be designed to, we'll come up with a design brief where we describe that and the technical basis for it and how we're going to evaluate the system and then submit that to the authority.

00:34:35.217 --> 00:34:54.346
<v Jonathan Hodges>Typically we'll have a meeting with them to also talk about it and talk through it, and the difficulty you run into with that type of approach is a lot of times the authority isn't as knowledgeable on the topic as the people who are doing the analysis and so they're generally going to accept what the fire engineer is saying is reasonable.

00:34:54.346 --> 00:35:03.050
<v Jonathan Hodges>You'll get some who are very knowledgeable who may kick back on something, but that's where education of the different stakeholders is important.

00:35:03.050 --> 00:35:07.762
<v Jonathan Hodges>There's an SFBE guide on PBD for code officials.

00:35:07.762 --> 00:35:19.023
<v Jonathan Hodges>I think it was ICC and SFBE put out and I think there's a lot of good guidance in there on what to be asking for from a designer for you to be reviewing as a code official.

00:35:19.023 --> 00:35:28.748
<v Jonathan Hodges>I think educating AHJs on those types of resources before they're being consulted with on a specific project they're being consulted with on a specific project.

00:35:28.748 --> 00:35:34.159
<v Jonathan Hodges>But just educating the community on these resources so that everyone is more knowledgeable about it, I think, is how we can overcome that obstacle.

00:35:34.869 --> 00:35:36.916
<v Wojciech Wegrzynski>And those credible FHIR scenarios.

00:35:36.916 --> 00:35:37.599
<v Wojciech Wegrzynski>Where do you get them?

00:35:37.599 --> 00:35:40.177
<v Wojciech Wegrzynski>I mean, there are resources that I know.

00:35:40.177 --> 00:35:48.085
<v Wojciech Wegrzynski>There's NIST database that I had Matt Bundy in the podcast where we've talked about how it was created.

00:35:48.085 --> 00:35:53.195
<v Wojciech Wegrzynski>There are other databases UL has or FSRI has their own database.

00:35:53.195 --> 00:36:01.596
<v Wojciech Wegrzynski>There's NFPA 204 Annex with my favorite stacks of palettes for specific feeds which no one understands in here.

00:36:01.596 --> 00:36:12.061
<v Wojciech Wegrzynski>A lot of sources, but you mentioned something like scaling experiments, so I'm really curious when do you get your design fires and and how you use experiments to get them?

00:36:12.371 --> 00:36:16.601
<v Jonathan Hodges>So I think the example I'll start with is let's talk about Christmas trees.

00:36:16.601 --> 00:36:20.179
<v Jonathan Hodges>Okay, so there's lots of data you can find.

00:36:20.179 --> 00:36:23.317
<v Jonathan Hodges>You can look at the SFB handbook that has.

00:36:23.317 --> 00:36:25.161
<v Jonathan Hodges>I'm looking at the chapter now.

00:36:25.161 --> 00:36:28.179
<v Jonathan Hodges>You've got a three megawatt peak for one of the trees here.

00:36:28.179 --> 00:36:31.032
<v Jonathan Hodges>You've got a 1.5 megawatt for another one.

00:36:31.032 --> 00:36:33.213
<v Jonathan Hodges>You can find lots of data on these.

00:36:33.213 --> 00:36:42.958
<v Jonathan Hodges>But most of these trees are about three meters tall, plus or minus maybe half a meter or something.

00:36:42.958 --> 00:36:46.942
<v Jonathan Hodges>You're in a mall or a hotel and they've got a five meter tall tree.

00:36:46.942 --> 00:36:50.003
<v Jonathan Hodges>How do you now scale up that measurement?

00:36:50.003 --> 00:37:01.594
<v Jonathan Hodges>Because it's not a linear scaling in terms of your fuel loading, because you've got generally a cone shape and so the fuel load density does not scale linearly with height.

00:37:01.614 --> 00:37:05.043
<v Jonathan Hodges>It scales with the volume of the cone or the density of your foliage or whatnot.

00:37:05.043 --> 00:37:11.900
<v Jonathan Hodges>So if you're going to scale that up, I would say at the very least you should be scaling it by combustible mass At least.

00:37:11.900 --> 00:37:14.692
<v Jonathan Hodges>Then you're doing some linear scaling there rather than by height.

00:37:14.692 --> 00:37:22.333
<v Jonathan Hodges>A six foot versus an eight foot tree I don't expect it to burn 20% faster, I expect it to burn more than that.

00:37:22.333 --> 00:37:32.864
<v Jonathan Hodges>So I think there's a lot of things you can talk about in terms of design fires, but I think the tree is a good example, because people can visualize a cone and say, okay, yeah, but the height doesn't really make sense.

00:37:32.864 --> 00:37:43.503
<v Jonathan Hodges>And so what we like to do is look at the data sources you mentioned the SFB handbook, fsri's database, fire calorimetry database that NIST puts out.

00:37:43.503 --> 00:37:46.199
<v Jonathan Hodges>All of those are good sources to find that starting point.

00:37:46.199 --> 00:37:51.940
<v Jonathan Hodges>But then we need to understand is that test data actually representative of what we have?

00:37:52.420 --> 00:37:53.302
<v Jonathan Hodges>You can have a couch.

00:37:53.302 --> 00:38:00.719
<v Jonathan Hodges>That's fine, and usually when people are burning couches, they're burning couches with kind of the worst type of cushion, the worst type of upholstery.

00:38:00.719 --> 00:38:03.139
<v Jonathan Hodges>So it's probably a conservatively bounding couch.

00:38:03.139 --> 00:38:05.336
<v Jonathan Hodges>But what if you have a sectional?

00:38:05.336 --> 00:38:12.498
<v Jonathan Hodges>You've got multiple, you've got two or three couches together and typically you've also got like a coffee table or something.

00:38:12.498 --> 00:38:13.981
<v Jonathan Hodges>Think about a hotel lobby.

00:38:13.981 --> 00:38:17.978
<v Jonathan Hodges>You might have two couches and a loveseat and a coffee table there.

00:38:17.978 --> 00:38:24.362
<v Jonathan Hodges>Do you need to consider all of those burning or do you just consider the kind of the one couch burning as a design scenario?

00:38:24.362 --> 00:38:43.780
<v Jonathan Hodges>And my argument would be that we need to consider those realistic configurations and then also, for each element within that realistic configuration, understand is that data appropriate or do we need to scale it up in some way, either by combustible mass or exposed surface area or different types of materials, etc.

00:38:44.402 --> 00:38:47.130
<v Wojciech Wegrzynski>What if you put that in a different setting?

00:38:47.130 --> 00:38:52.847
<v Wojciech Wegrzynski>That it was tested in the way how you would burn a Christmas tree under a hood is exactly what I've just said.

00:38:52.847 --> 00:38:55.434
<v Wojciech Wegrzynski>You have a hood and you burn the Christmas tree underneath it.

00:38:55.434 --> 00:39:07.378
<v Wojciech Wegrzynski>There's no sitting in between them, there's no walls that prevent the smoke flying sideways, there's no large opening five meters from the tree from which you might have a three meter per second flow on the tree.

00:39:07.378 --> 00:39:21.833
<v Wojciech Wegrzynski>Do you have any approach to scale the design fire to the circumstances in which it will be used, or you would still just go with the measurements from the lab because you don't want to make your own source of uncertainty in that?

00:39:22.195 --> 00:39:25.914
<v Jonathan Hodges>So I think we would typically use the free burning results and scale them up.

00:39:26.255 --> 00:39:42.773
<v Jonathan Hodges>But I do see a place for fire growth modeling and understanding those other effects If you calibrate a model to where it's matching the experiments and you've scaled it up and then looking at some of these other geometric effects to understand if they make it worse.

00:39:43.373 --> 00:40:06.384
<v Jonathan Hodges>There's a few studies on heavy good vehicles in tunnels, for example, where they look at the cross-sectional area of the tunnel and there's kind of a sweet spot in that where you get this balancing of fresh air coming in, accelerating combustion but also retaining enough heat that you're getting enough pyrolysis to really support those huge fires that you see.

00:40:06.384 --> 00:40:12.050
<v Jonathan Hodges>And then if you make it too big, then you're not retaining as much heat even though you're making more oxygen available.

00:40:12.050 --> 00:40:22.525
<v Jonathan Hodges>And if you get it too small you don't have enough oxygen available or your flow rate's so high that you end up cooling your gases a lot and you're not really retaining that heat.

00:40:22.525 --> 00:40:34.362
<v Jonathan Hodges>So developing a model that can predict and experiment and then using that in these different scenarios to understand design implications, I think is a very important step of where we're going as a community.

00:40:35.170 --> 00:40:52.063
<v Wojciech Wegrzynski>So the tunnels is an interesting one because there were also research like if you put a tarp on the pallets on the truck, it changes the heat release rate of the fire, which is exactly what I've asked about, meaning that you changed the environment in which the fire takes place.

00:40:52.063 --> 00:40:55.480
<v Wojciech Wegrzynski>It already affects it significantly.

00:40:55.480 --> 00:41:01.583
<v Wojciech Wegrzynski>What about trying to do reduced-scale experiments and scale up from those?

00:41:01.583 --> 00:41:03.231
<v Wojciech Wegrzynski>Do you have any experiences with that?

00:41:03.231 --> 00:41:10.487
<v Wojciech Wegrzynski>They're the only ones that we're ever using are again related to tunnel scenarios.

00:41:10.487 --> 00:41:17.943
<v Wojciech Wegrzynski>There are some data on trains in 1 to 3 scale from RISE, but I'm not really fond of scale modeling.

00:41:17.943 --> 00:41:22.994
<v Wojciech Wegrzynski>I wonder what's your opinion on that source of knowledge for design fire information.

00:41:23.516 --> 00:41:26.726
<v Jonathan Hodges>So we've done a lot of work on scaling rail car fires.

00:41:26.726 --> 00:41:39.065
<v Jonathan Hodges>So we've tested some quarter scale and three eight scale rail cars in our lab and looked at how well the scaling laws hold across scales.

00:41:39.065 --> 00:41:42.152
<v Jonathan Hodges>I think it's a good approach in some cases.

00:41:42.152 --> 00:41:48.547
<v Jonathan Hodges>But the difficulty is in fire you can't, you can never preserve all your dimensionless groups.

00:41:48.547 --> 00:41:57.525
<v Jonathan Hodges>In a typical environment you're going to be just looking at preserving Froude number and so that's going to be your heat release rate to the two-fifth power.

00:41:57.525 --> 00:42:02.255
<v Jonathan Hodges>But that's only preserving the fire plume because that's what it's designed around.

00:42:02.255 --> 00:42:30.597
<v Jonathan Hodges>But when you look at Froude modeling you're making the assumption that time scales I think it's timed scales with length ratio to the one half power or something and that leads to all sorts of difficulties when you start looking at flame spread, where your heat release rate really scales by length to the second power, because if you've got the same thermal environment then you would expect your heat release rate per unit area to be constant.

00:42:30.597 --> 00:42:41.608
<v Jonathan Hodges>And so you get into these difficulties where now you need to be scaling your burning rate because your time is scaling, but physically that doesn't happen with fluid modeling.

00:42:41.608 --> 00:42:44.371
<v Jonathan Hodges>So you get into these difficult places.

00:42:46.514 --> 00:42:57.451
<v Jonathan Hodges>I think it's a good starting point and with what we've looked at is really in post-flashover fires where you have mostly kind of a one zone environment, that scaling with Q to the second power.

00:42:57.451 --> 00:43:09.141
<v Jonathan Hodges>So just looking at maintaining the exposed surface area of combustibles and saying that that should be burning at the same rate, that seems to do a pretty good job in these cases.

00:43:09.141 --> 00:43:16.440
<v Jonathan Hodges>But the growth to flashover is not preserved in that and you have to the way you handle your ventilation.

00:43:16.440 --> 00:43:29.701
<v Jonathan Hodges>You can't linearly scale your ventilation size because your opening factor derives what that one layer temperature is and so you need to preserve that across scales.

00:43:29.701 --> 00:43:38.019
<v Jonathan Hodges>So you have to relax your geometric similarity if you're using that type of approach, but in a post-flashover tends to work pretty well.

00:43:39.063 --> 00:43:52.186
<v Wojciech Wegrzynski>And what about approaches where you would take some very small scale material scale data imagine a cone colorimetry and try to scale that up into a design fire in your CFD real-world full-scale scenario?

00:43:52.186 --> 00:43:55.826
<v Wojciech Wegrzynski>Any experiences, any opinions on that as a part of engineering?

00:43:56.315 --> 00:43:58.884
<v Jonathan Hodges>So that's something that we do quite frequently as a company.

00:43:59.536 --> 00:44:29.126
<v Jonathan Hodges>We'll test materials in our lab and this goes into the fire growth discussion that we had earlier, where you're taking that cone data, putting it into the model, looking at your flame thread rates, looking at your heat release rate, and we're using that really to inform what the design scenario should be, more so than just as its own parameter, because you do start to get your uncertainties stacked on top of each other, like some of the work we're doing right now looking at heat fluxes to surfaces.

00:44:29.126 --> 00:44:32.454
<v Jonathan Hodges>Heat fluxes are not grid independent.

00:44:32.454 --> 00:44:36.764
<v Jonathan Hodges>When you've got a fire impinging on the surface, lots of reasons for that.

00:44:36.764 --> 00:44:48.126
<v Jonathan Hodges>But that means that as you refine your grid more and more, your heat flux is changing more and more and more, and that's going to affect the answer you get out in fire growth, and it's not necessarily.

00:44:48.126 --> 00:44:53.027
<v Jonathan Hodges>You know you have to get down to about a one millimeter resolution before it really starts to converge.

00:44:53.027 --> 00:45:00.409
<v Jonathan Hodges>But even then the number that you're converging to might not be representative of reality, because it depends.

00:45:00.409 --> 00:45:05.206
<v Jonathan Hodges>If you're using infinitely fast chemistry you're at a one millimeter grid resolution.

00:45:05.206 --> 00:45:12.056
<v Jonathan Hodges>That might not really be how it would burn in reality, and so that's where we're working on improving those things.

00:45:12.317 --> 00:45:19.222
<v Jonathan Hodges>But when you're dealing with test data and trying to plug it into a model to use that to come up with a design scenario you need to.

00:45:19.222 --> 00:45:24.184
<v Jonathan Hodges>Really, this is where the fire science and the understanding of how things burn really comes in.

00:45:24.184 --> 00:45:52.096
<v Jonathan Hodges>I would rather have somebody who understands fire testing and has read through these intro and fundamentals fire dynamics books and really understands fire and have them work on this on a CFD project with me, Then someone who has never worked in fire but has done a lot of CFD modeling, who doesn't understand these things because I can work with them on the CFD side as well.

00:45:52.096 --> 00:45:55.983
<v Jonathan Hodges>But it takes a lot of experience to learn the fire science part.

00:45:56.445 --> 00:45:57.467
<v Wojciech Wegrzynski>Absolutely so.

00:45:57.467 --> 00:46:11.257
<v Wojciech Wegrzynski>In your case, the cone would be a part of this refinement loop, like informing your further refined design fire choices, the spread model that allows you to approximate and scale up the experimental results.

00:46:11.257 --> 00:46:14.681
<v Wojciech Wegrzynski>If seeking for a design fire, you have a number for a three meter tree.

00:46:14.681 --> 00:46:16.465
<v Wojciech Wegrzynski>You would use those to inform your decision.

00:46:16.465 --> 00:46:18.557
<v Wojciech Wegrzynski>How a five meter tree?

00:46:18.878 --> 00:46:21.905
<v Jonathan Hodges>Okay, so we do cone calorimeter experiments.

00:46:21.905 --> 00:46:28.007
<v Jonathan Hodges>We'll typically do three heat fluxes and we have the scaling pyrolysis model that we've put in FDS now.

00:46:28.007 --> 00:46:37.563
<v Jonathan Hodges>It allows you to take the heat flux, cone data say 50 kilowatt per meter squared and dynamically scale it up or down based on the incident heat flux.

00:46:37.563 --> 00:46:44.922
<v Jonathan Hodges>So we'll use that pretty regularly when we're looking at design fires right now and trying to understand how things could burn.

00:46:45.382 --> 00:46:45.925
<v Wojciech Wegrzynski>Fantastic.

00:46:45.925 --> 00:46:49.606
<v Wojciech Wegrzynski>Okay, this went fast the interview.

00:46:49.606 --> 00:46:59.065
<v Wojciech Wegrzynski>Perhaps for the closing remark, one thing that excites you in this world what do you see for the near future or maybe far future?

00:46:59.065 --> 00:47:05.500
<v Wojciech Wegrzynski>How do you think the profession, from this point that we've discussed CFD, design fires, how we use those tools.

00:47:05.500 --> 00:47:10.545
<v Wojciech Wegrzynski>Do you see any shifts coming that engineers should get excited for or scared about?

00:47:10.974 --> 00:47:14.945
<v Jonathan Hodges>So one thing that I'm excited about is just how far we've come in recent years.

00:47:14.945 --> 00:47:16.315
<v Jonathan Hodges>You think about it.

00:47:16.315 --> 00:47:23.916
<v Jonathan Hodges>The first public release of FDS was about 20 years ago, and now you see how broadly used it is in our industry.

00:47:23.916 --> 00:47:36.204
<v Jonathan Hodges>But in the scale of construction, design and engineering, that's not a long time, and so we're still refining the tool, learning more about the physics of what we're doing.

00:47:36.204 --> 00:47:42.887
<v Jonathan Hodges>But we've come a long way and I'm excited to see where we're going to be coming in the next 10, 20 years.

00:47:42.887 --> 00:48:01.644
<v Jonathan Hodges>You'd mentioned AI and leveraging AI-driven CFD surrogate modeling or AI-driven subphysics models in CFD, and I see a lot of potential for those going forward to improve the fidelity of the tools while not having to sacrifice on the computational time.

00:48:02.496 --> 00:48:11.405
<v Wojciech Wegrzynski>I'm super excited about the growth in the computational space, like the GPU solvers and everything that's related to that.

00:48:11.405 --> 00:48:18.469
<v Wojciech Wegrzynski>We're really almost at a point where you will be able to do your CFDs like we're doing zone model today.

00:48:18.469 --> 00:48:34.983
<v Wojciech Wegrzynski>Some years ago I thought it's not going to ever scale up to this level, but today I see perhaps it's not yet this generation of GPU-based solvers, but the next one for sure, where we're going to beat the real-time CFD.

00:48:34.983 --> 00:48:43.385
<v Wojciech Wegrzynski>We're almost at this edge right now, so we're definitely going to beat that and it's going to open a lot of new opportunities.

00:48:43.385 --> 00:48:49.563
<v Wojciech Wegrzynski>Like even in this discussion, we talked about why you need to choose the credible scenario because you cannot run a hundred right.

00:48:49.563 --> 00:48:49.945
<v Wojciech Wegrzynski>What if you could?

00:48:49.945 --> 00:48:50.710
<v Wojciech Wegrzynski>Like that cannot run a hundred right.

00:48:50.710 --> 00:48:51.275
<v Wojciech Wegrzynski>What if you could?

00:48:51.275 --> 00:48:53.681
<v Wojciech Wegrzynski>That would be fun world right.

00:48:54.023 --> 00:48:59.206
<v Jonathan Hodges>Yeah, and NIST is doing a lot of really interesting work on the GPU computing with FDS.

00:48:59.206 --> 00:49:04.246
<v Jonathan Hodges>Now there's a research fork that was presented at the FM workshop.

00:49:04.246 --> 00:49:12.632
<v Jonathan Hodges>On it we're presenting and showing that you can get some GPU acceleration now with FDS, that you can get some GPU acceleration now with FDS.

00:49:12.632 --> 00:49:19.197
<v Jonathan Hodges>Eric also presented on a case for the wildland fire simulation with FDS that used 1.3 billion cells, Like it's.

00:49:19.197 --> 00:49:26.119
<v Jonathan Hodges>We're at the place where you've got if you've got enough computing horsepower, you really can throw a lot of computing power at these tools now.

00:49:26.119 --> 00:49:31.179
<v Jonathan Hodges>But on the flip side, if you give a scientist more computing power, they'll find a way to use it.

00:49:31.179 --> 00:49:41.590
<v Jonathan Hodges>There's always going to be more you can do in your chemistry in the gas phase or more radiation angles that you can do more detailed chemistry or thermal.

00:49:41.590 --> 00:49:45.425
<v Jonathan Hodges>So there's always going to be more you can do with it.

00:49:45.425 --> 00:49:50.849
<v Jonathan Hodges>But I'm excited about where we're going to be in the following years about where we're going to be in the following years.

00:49:54.574 --> 00:49:59.822
<v Wojciech Wegrzynski>Let's say, a funny observation was that the computational demands of FDS were growing faster than the computational power growth, but I think the trend will reverse.

00:49:59.822 --> 00:50:06.815
<v Wojciech Wegrzynski>I think, yeah, it's the problems you've mentioned Us wanting to do more and more and more complicated analysis.

00:50:06.815 --> 00:50:08.262
<v Wojciech Wegrzynski>That's going to be a limiting factor.

00:50:08.262 --> 00:50:14.362
<v Wojciech Wegrzynski>I'm all into more scenarios and I'm really excited for what the future will bring, Jonathan, thank you.

00:50:14.362 --> 00:50:16.864
<v Wojciech Wegrzynski>Thank you so much for coming to the Fire Science Show.

00:50:16.864 --> 00:50:21.782
<v Wojciech Wegrzynski>See you around, hopefully soon somewhere in the world.

00:50:22.195 --> 00:50:22.556
<v Jonathan Hodges>Sounds good.

00:50:22.556 --> 00:50:27.460
<v Jonathan Hodges>Are you going to be able to make it to the annual meeting in Louisville, or are you not going to be able to make it this year?

00:50:27.914 --> 00:50:29.659
<v Wojciech Wegrzynski>No, this year is New zealand for me.

00:50:29.659 --> 00:50:34.568
<v Wojciech Wegrzynski>I'm sorry, tough choice, not really that'd be fun.

00:50:34.608 --> 00:50:43.306
<v Wojciech Wegrzynski>I've not had the chance to go to new zealand before my new zealand audience, let's have a beer, and for my us audience, there's gonna be an opportunity in the near future.

00:50:43.306 --> 00:50:45.699
<v Wojciech Wegrzynski>Anyway, near is up to a definition.

00:50:45.699 --> 00:50:47.344
<v Wojciech Wegrzynski>Thanks, jonathan, and that's it.

00:50:47.344 --> 00:50:48.105
<v Wojciech Wegrzynski>Thank you for listening.

00:50:48.105 --> 00:50:59.425
<v Wojciech Wegrzynski>I hope you've enjoyed the two practitioners view on how cfd is used and I think Jonathan has brought us a lot of new, interesting insight in how this work is carried worldwide.

00:50:59.425 --> 00:51:10.625
<v Wojciech Wegrzynski>Jensen Kings is certainly a big company offices all around the world so it must be interesting to practice CFD and fire safety engineering in such a big, massive group.

00:51:11.007 --> 00:51:28.822
<v Wojciech Wegrzynski>My takeaway is that the distinction between the CFD that we can use to assess the consequences of fires and predicting the growth of fires, fire spread simulations these are two completely different things, different regimes of modeling, different challenges and different uncertainties.

00:51:28.822 --> 00:51:48.327
<v Wojciech Wegrzynski>I appreciate Jonathan's insight in how one can venture a little beyond just modeling the consequences, how you can improve your design fires by iterative modeling and how getting data from the real world, from cone, from laboratories, from full-scale burns, how this can all help us design our fires better.

00:51:48.327 --> 00:51:51.525
<v Wojciech Wegrzynski>Because in the end, you know, cfd is a sophisticated methodology.

00:51:51.525 --> 00:52:13.923
<v Wojciech Wegrzynski>It's a sophisticated tool that we all use in fire safety engineering, but the tool is only as good as the stuff that you put inside, and if the design fire is the most dominant thing in the entirety of the design, no matter how complicated your model is, it's the design fire that makes or breaks your simulation, so emphasizing that, thank you, jonathan, for coming to the fire science show.

00:52:14.063 --> 00:52:15.148
<v Wojciech Wegrzynski>I hope you've enjoyed.

00:52:15.148 --> 00:52:16.673
<v Wojciech Wegrzynski>I'm a little tired.

00:52:16.673 --> 00:52:23.157
<v Wojciech Wegrzynski>We're having a blast at the summer school of fire fundamentals for performance-based design and it's a great time.

00:52:23.157 --> 00:52:28.568
<v Wojciech Wegrzynski>It was also very intense and I hope your week is also going great.

00:52:28.568 --> 00:52:36.588
<v Wojciech Wegrzynski>I've delivered to you a bit of fire science going back to my duties at the conference and will deliver you more next week.

00:52:36.588 --> 00:52:37.673
<v Wojciech Wegrzynski>Cheers, bye.